The Otto Cycle engine is a reciprocating internal combustion engine. Many of the key work-producing components of the Otto Cycle engine reciprocate, that is they are required to move in a first direction, stop, and then move in a second, opposite direction in order to complete the cycle. In the Otto Cycle engine, there are four changes of direction of the piston assembly in effecting a single power stroke. Piston assemblies (e.g., pistons, rings, wrist pins and connecting rods) travel up into their respective cylinders at a changing rate of speed to top dead center (i.e., to the end of the stroke), where they stop and then return down the cylinder to the bottom of the stroke. The connecting rod, traveling with the piston and articulating at the wrist pin and orbiting at the crankshaft presents a changing angular force that results in side loading of the piston against the cylinder wall. This causes frictional losses. Because of acceleration and deceleration of the piston components in their movements, the internal combustion reciprocating engine requires a flywheel to moderate these energy surges, but this is an imperfect solution and there remain energy-consuming effects.
The Otto Cycle engine also employs the piston/cylinder relationship to pump air into the cylinder (through reciprocating valves) to support combustion and then to pump the exhaust gases out of the cylinder through reciprocating valves. A significant amount of the engine power is used to achieve the pumping action and two revolutions of the crankshaft are required to effect one power stroke.